The valve spools of hydraulic control valves are commonly provided with metering slots or notches connecting an annular groove with a peripheral surface of an adjacent land. The slots provide a metering effect of the fluid flow therethrough and thus more precise control over the actuation of a hydraulic motor or jack than can be achieved by a simple spool shoulder. The two basic types of fluid flow through the slots are referred to as "meter in" wherein the fluid metered through the slots is in a direction toward the spool and "meter out" wherein the fluid metered through the slots is in a direction outwardly from the spool.
It is well known that the fluid flowing through a valve exerts a force against the spool. Although a mathematical analysis of the flow forces has been evasive, a substantial portion of the flow forces acting on the spool during a metering condition are attributed to the momentum force of the fluid. Momentum force can best be explained as the force exerted on the spool by the fluid when the spool changes the direction of fluid flow. The flow force increases with an increase in either velocity or pressure drop of the fluid. Heretofore, valve spools have been designed to neutralize the flow forces or to reduce the force to an acceptable level. It is found in practice that the end mill slot when used in a meter out condition and the key cutter slot when used in a meter in condition are effective in reducing the flow forces at high flows and pressures.
The two types of metering conditions i.e., meter in and meter out, usually present drastically different flow forces acting on the valve spool. For example, in a meter out condition, the fluid flow has always produced a flow force acting on the spool in a direction tending to close the metering slot regardless of the slot configuration. However, in a meter in condition, the flow force may either tend to close or open the metering slot. Thus depending on the particular metering situation, the flow force urges the valve spool either in the direction tending to close the metering slot or in the opposite direction tending to open the metering slot.
Some hydraulic control valves have a pump port disposed between a pair of tank ports with the valve spool normally communicating the pump port with both of the tank ports. To establish fluid pressure in the hydraulic system, the valve spool may be shifted in either direction to block the pump port from both of the tank ports and communicate the pump port with a motor port which is connected to a hydraulic motor or jack. In an effort to reduce the complexity of the valve body casting and to provide a valve which is more compact, one of the tank ports has been deleted in some valve designs so that the valves are provided with a pump port which is normally in communication with a single axially adjacent tank port through an annular groove of the valve spool. To establish fluid pressure in the hydraulic system for actuating a hydraulic motor or jack, the valve spool may be shifted in either direction to initially meter and eventually block fluid flow from the pump port to the tank port and to connect the pump port with one side of the hydraulic motor to communicate pressurized fluid thereto. Thus, in one direction of spool travel, a meter out condition exists while in the opposite direction of spool travel a meter in condition exists. In this environment and when the more common key cutter type metering slots are employed, the metering out condition produces a flow force acting on the spool in a direction tending to close the slot. Conversely, depending on the particular geometry of the slot, the meter in condition can produce a flow force acting on the spool tending to either open or close the metering slot.
When the above described type of valve design is employed in a manually operated system the same amount of lever movement by the operator will generate substantially equal flow in either direction of shift assuming equal load resistant upon the hydraulic jack. Different shifting effort will be required, but lever movement will be approximately the same.
However, a problem is encountered when that type of control valve is employed as the main control valve of a pilot operated system wherein the valve spool of the main control valve is actuated by fluid pressure. In such an arrangement, lever movement by the operator does not directly cause movement of the main control valve spool, but generates only a pressure output directed to the appropriate end of the valve spool. Thus, since the flow force against the main control spool can react in different directions, the same amount of lever movement of the pilot control valve does not always provide equal amounts of movement of the main control valve spool. This makes it difficult for the operator to judge how far to actuate the control lever to achieve a particular speed of the hydraulic motor or jack.